This invention relates to nonlinear frequency conversion of infrared laser sources and in more detail to the use of molecular liquids as the active media in two-photon resonant four-wave mixing processes to achieve efficient nonlinear conversion of infrared laser frequencies.
All laser applications have some degree of wavelength dependence which can be optimized by varying the laser frequency. Thus, for example, military and communications applications depend upon atmospheric transmission characteristics which are highly wavelength dependent; laser photochemistry applications require very precise tuning of the laser frequency to atomic and molecular resonances. Existing sources cannot meet all of these varying wavelength requirements. Nonlinear mixing techniques in which existing laser frequencies are combined are a useful means of generating coherent light sources at new frequencies which are not presently available from primary laser sources.
Crystals used to generate sum and difference frequencies between two laser frequencies have been the most commonly used nonlinear elements to date. For many potential applications these crystals are inadequate because of limited size, poor crystal quality and low breakdown thresholds.
Since liquids and gases are isotropic media, they cannot be used in three-wave processes such as sum and difference frequency generation. However, higher order nonlinear processes are possible and, in particular, four-wave processes in which three laser frequencies are combined to generate a fourth frequency are the lowest order nonlinear processes allowed by symmetry considerations in such isotropic media. If .omega..sub.1, .omega..sub.2 and .omega..sub.3 are used to designate the three input frequencies to the nonlinear device then the most general expression for the output frequency is .omega..sub.4.+-..+-. = .vertline..omega..sub.1 .+-. .omega..sub.2 .+-. .omega..sub.3 .vertline..
Such four-wave interactions have previously been demonstrated in solids and gases in the infrared spectral region. The efficiencies, however, particularly for gases have been too low to allow construction of practical devices. The highest reported efficiency for third harmonic generation (THG) of CO.sub.2 laser radiation, .omega..sub.4 = 3.omega..sub.1, is 10.sup.-6 using the crystal CdGeAs.sub.2. Only one previous report of a four-wave process in a liquid medium has been given; this was a THG process in a room temperature organic liquid pumped with a 1.06 .mu.m input laser wavelength. The efficiency was not reported but was presumably quite low. The efficiencies of four-wave mixing processes can be substantially enhanced by making use of intermediate state resonances. This approach has been demonstrated in the infrared for THG in CO gas at pressures up to several atmospheres using the CO.sub.2 pump lines R(8) and R(10) at 9.3 .mu.m which are in two-photon resonance with the vibrational-rotational lines of the CO Q-branch transitions at 4.7 .mu.m. However, the best reported efficiency is only 10.sup.- 8, still too low to be a practical device.
It is therefore an object of the invention to provide a more efficient nonlinear frequency converter for laser frequencies.
It is a further object of this invention to provide this function in a relatively compact device.
Features of the invention include the ability to recover from any optically induced breakdown without degradation of the device function and the ability to flow the active medium in high average power applications.